This invention relates to a mirror of the class constituted fundamentally of a hard and transparent synthetic resin plate and a thin reflective film of a metal coated on one side of the resin plate. For example, a mirror according to the invention is suitable for use as a rearview mirror in automobiles or other vehicles.
At present almost every mirror is produced by coating one side of a glass plate with a thin film of aluminum, which is a material very high and almost comparable to more expensive silver in reflectivity. For practical mirrors it is necessary to cover the reflective film of aluminum with a protective back-coat layer. One of the purposes of this back-coat layer is to prevent exposure of the reflective film to humidity because aluminum in the form of thin film is easily eroded by moisture. Since glass plate is impermeable and insusceptible to humidity, it is easy to obtain a sufficiently durable mirror by ensuring the humidity resistance of the back-coat layer.
However, durability becomes a matter for serious consideration when it is intended to produce a plastic mirror by utilizing a synthetic resin that is hard, transparent and high in optical clarity as typified by polymethyl methacrylate resin in place of glass. It is not so difficult to deposit a thin film of aluminum on one side of a plate of such a resin, and an impermeable back-coat layer necessary to the aluminum film can be formed with little difficulty. The problem arises from the fact that synthetic resins useful as mirror body material are generally hydroscopic to some extent. The transparent plate of a plastic mirror gradually absorbs moisture from the atmosphere and as a consequence undergoes swelling, but because of the existence of the impermeable back-coat layer on the back side the swelling of the resin plate occurs exclusively on the front side. Therefore, gradually the resin plate warps to have an irregularly convex front surface. Of course such deformation or distortion of the mirror body means serious degradation of the quality of the mirror.
Since the resin plate of a plastic mirror is liable to suffer scratches, it becomes necessary to coat the front surface of the resin plate (optionally the other surfaces, too) with a transparent surface-hardening film by using, for example, an organosilicon polymer as the coating material. The above explained problem arisen from hygroscopicity of the resin will dissolve if this surface-hardening film can be made impermeable to humidity. Actually, however, no currently available surface-hardening coating material for synthetic resins can give a film sufficiently high in both transparency and impermeability to humidity.
Meanwhile, there is an eager demand for fully practical plastic mirrors in various fields. In automobiles industries, for example, a plastic inside rearview mirror has been considered to be advantageous in its nonbreakableness that is favorable to the safety of the driver in case of an accident and also in its light weight that contributes to the reduction of the gross weight of the car. In some cases where the mirror body has a relatively intricate shape as exemplified by a cross-sectionally wedge-like shape of a usual inside rearview mirror, even a reduction of the production cost can be expected by the employment of a synthetic resin mirror body which can be easily be shaped by a method high in productivity, such as injection molding, without the need of any machining operation. Until now, however, the demand has not yet been met mainly by reason of the above described problem.